1. Field of the Invention
The present invention relates generally to methods and apparatus for optical beam induced current investigation in integrated circuits, and more particularly to a method and apparatus for optical beam induced current investigation that requires only one contact with the circuit through use of intrinsic system capacitance as an electrical return path.
2. Brief Description of the Prior Art
It is well known in the art that if an optical beam impinges on a semiconductor junction, a current will be generated by the junction. The current is due to a built-in potential in every semiconductor junction as a result of band bending. This phenomena has been used extensively to image semiconductor junctions in the areas of general semiconductor device analysis and semiconductor failure analyses. Prior art methods require the connection of both ends of a junction to a direct current/low frequency current detection and processing circuitry. Due to the difficulty in making these connections on the interior of an integrated circuit, it is impractical or impossible to analyze junctions that do not have both ends connected to lines leading to external contacts.
The problem is illustrated in FIG. 1. An integrated circuit 10 is shown with two diodes 12 and 14 symbolically represented. Diode 12 has both ends connected to lines leading to external contacts 16 and 18 that can be easily accessed for analysis. In contrast, the ends of diode 14 do not connect to an xe2x80x9cexternalxe2x80x9d contact. In order to analyze diode 14, prior art methods require the use of probes 20 and 22. Positioning these probes is a very precise and time consuming operation and therefore undesirable.
FIG. 1 also illustrates the prior art process of imaging. An optical beam 24 of constant signal strength is shown impinging on diode 14. In response, diode 14 generates a current that flows through conductor 26 to the current amplifier circuit 28 and back through return line 30. The output 32 of the amplifier 28 is then received by the signal processor 34 wherein the signal is detected, and in response the processor provides an output indicative of the current and corresponding junction status.
From the above description of the prior art, it is apparent that there is a need for an improved method and apparatus for imaging junctions that are located on the interior of an integrated circuit.
It is therefore an object of the present invention to provide an improved method and apparatus for performing optical beam induced current imaging of semiconductor junctions.
It is a further object of the present invention to provide a method and apparatus for performing optical beam induced current imaging of semiconductor junctions located interior on integrated circuits without the need for probing of interior portions of the circuit.
It is a still further object of the present inventions to provide an improved method and apparatus for performing optically induced current imaging of semiconductor junctions that require only a single wired contact with the integrated circuit.
Briefly, a preferred embodiment of the present invention includes an improved method of performing optical beam induced current imaging of semiconductor junctions. According to the method, a single wired contact to an integrated circuit (for example through use of a conventional conductive probe) is made at a point that makes electrical contact to a first side of a junction to be analyzed. A first line connects the wired contact to an amplifier, and a second line carries return current from the amplifier to a ground connection. A capacitive return connection is then used to couple return current from a second side of the junction to ground. The actual value of capacitance in the return connection is not of fundamental importance in the practice of the invention, but generally larger capacitance values are preferred and allow increased induced current flow. With the connections described above to the amplifier and the processing circuitry, a pulsed optical beam is applied to the junction. In response to the pulsed beam, the junction generates a corresponding pulse of induced current. Since the current in response to the pulsed beam has an alternating current component, it passes through the capacitive junction return connection as well as through the single wired contact to the amplifier. The amplifier outputs a magnified replica of the current which is then analyzed by the processing circuit to provide a useful form of data for analysis of the junction.
An advantage of the present invention is that it provides a method and apparatus for rapid performance of optical beam induced current imaging of semiconductor junctions.
Another advantage of the present invention is that it avoids the need to probe individual junctions on an integrated circuit in order to perform optical beam induced current imaging.
A further advantage of the present invention is that it substantially reduces the labor and corresponding cost of performing optical beam induced current imaging of semiconductor junctions on an integrated circuit.